feat: built root and user docker image variants

[drbh]

This PR improves the docker built in various ways

1. split dockerfile into root and non root builds
   • root for dev/ when the user has control over container launch
   • non root that expect uid 1000 to lock down permissions in CI/non rootless container envs
2. bump workflow to publish both variants of containers
3. prefer the --mount flag over the older --volume when launching the container for more granular permission control`

[drbh]

example usage

git clone  https://github.com/huggingface/kernel-builder
cd kernel-builder
docker build -t kernel-builder:local -f dockerfiles/Dockerfile .

git clone https://huggingface.co/medmekk/rmsnorm_thrust
cd rmsnorm_thrust

then run the builder container build

docker run --mount type=bind,source=$(pwd),target=/kernelcode kernel-builder:local
# ...
# rmsnorm_kernel-torch-ext> ✅ No compatibility issues found
# building '/nix/store/1w31r5rj5d1f8pp0waamza4c72mlbqcm-torch-ext-bundle.drv'...
# Build completed. Copying results to /kernelcode/build
# Done - results available in /kernelcode/build